In general, looking across the three assessments, the areas variable property of an experimental subject and measurement of outcome were easier for most students at the end of the course than manipulation of variables, accounting for variability, or scope of inference.
Similarly, for the bird assessment, the largest differences in the proportion of students with difficulties were found for the areas measurement of outcome and manipulation of variables, while difficulties involving accounting for variability and scope of inference remained almost unchanged at the end of semester (medium-gray bars in Figure 2, E and F).
For example, it is not surprising that a difficulty with controlling outside variables categorized under manipulation of variables was associated with difficulty accounting for variability, because controlling outside variables provides a way to account for and minimize natural variation in samples.
This process led to the development of the RED (Table 2) with five major categories of student difficulties with experimental design as themes: 1) variable property of an experimental subject; 2) manipulation of variables; 3) measurement of outcome; 4) accounting for variability, and 5) scope of inference based on the findings.
Consistently, a careful analysis of responses revealed difficulties with experimental design in five areas: 1) a property of an experimental subject that is variable, 2) manipulation of variables, 3) measurement of outcome, 4) accounting for and measuring variability, and 5) scope of inference of findings.
Data collected from undergraduate biology students, together with difficulties data from a review of the literature, confirmed five major areas of difficulty with experimental design: 1) a property of an experimental subject that is variable; 2) manipulation of variables; 3) measurement of outcome; 4) accounting for and measuring variability; and 5) scope of inference of findings.
Similar manipulations of variables including motivational state, reward value (e.g., regular versus palatable food), predictor cues, and the costs and/or predictability of obtaining rewards through different reinforcement schedules might be used to investigate the variables that determine changes in cortical excitability associated with food reward [ 24, 26, 31, 33, 34].
A simple 2k factorial design has shown that a very significant increase in the regioselectivity of vinyl-aromatics (typically from 70% to more than 98%) can be obtained by the exclusive manipulation of two variables, pressure and temperature, within the usual working values for such variables.
The manipulation of these variables was found to affect the PSD significantly.
Preliminary evidence suggests that the reactor could eventually lead to the ability to control particle properties, such as size, composition and shape, through the manipulation of process variables.
Laboratory experiments provide an alternative that allows manipulation of environmental variables and simulation of altered conditions that are expected in the future.
